These photographs, taken at the Naval Research Laboratory in Washington, D.C., show the forearms of several test subjects after they were exposed to nitrogen mustard and lewisite agents during World War II.

Courtesy of the Naval Research Laboratory

Decades after the U.S. government exposed service members to chemical weapons in secret experiments, lawmakers have advanced a measure intended to make it easier for those World War II veterans to obtain compensation. The bill, known as the Arla Harrell Act, advanced to President Trump's desk after Senate approval Wednesday.

"When a Missouri veteran is mistreated, I take it personally — and I'll take the fight to anyone, anywhere, to make it right," Sen. Claire McCaskill said in a statement, referring to the namesake of the bill she sponsored. The Missouri Democrat named the bill for one of her constituents, a veteran who says he was one of the 60,000 American test subjects exposed to mustard gas and lewisite agents by the U.S. government during the war.

"After all these years," McCaskill added, "it's frankly less about the benefits that Arla deserves, and will now receive — it's about recognizing what he sacrificed for this country, and that he and his family deserve to hear three simple words from their government. We believe you."

Of the 4,000 men the department had sought to locate — the men who were exposed to the most extreme experiments — officials said they found and attempted to reach only 610 in the span of more than two decades.

McCaskill introduced the Arla Harrell Act twice — both last year and again at the start of this year, when the new Congress gathered in January. But it was not until the measure was appended to a veterans-benefits package that it made an appreciable move forward.

The proposed change is similar to a 1991 law that facilitated claims by Vietnam War veterans who were exposed to Agent Orange. In that situation, as with the secret mustard gas tests, veterans struggled to provide the VA with enough evidence to qualify for compensation.

"I'm delighted that this got passed by Congress," Veterans Affairs Secretary David Shulkin told reporters Thursday. "We are deeply appreciative of the 400 or so veterans that we believe have been waiting too long to be recognized for what they deserve. This is really what allows the VA to be able to move forward in doing the right thing."